Quote:
Originally Posted by Upgrade+
Glad you posted this. I'm having the same issue with my '11 135i. It usually happens when I come to partial stop.
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DCT? If not on a DCT then don't bother reading the rest as it probably means I am completely wrong LOL
I have the same thing, but I am pretty sure its because of the DCT, not because of engine or any other kind of lag. The brain for the tranny thinks that you are about to come to a complete stop based on the deceleration rate and current rpm so it begins to disengage the clutches altogether. If you don't manually downshift it to 1st prematurely while slowing down, the tranny would actually disengage from 2nd into neutral which means the first pack is still hooked to 3rd, the 2nd pack to 1st. If right at that time you hit the gas the tranny has to change the first pack to 1st, and then engage the clutch. Just this alone could take almost an entire second and if the hitting of the gas happens right when the disengage of both clutches is signaled then you have some extra lag to complete the disengage, plus change pack 1 into right gear, plus engage again.
You can try a couple of things to measure the lag of the tranny in these transition phases:
1. Manually downshift to first while slowing down. This will help quite a bit, but there will still be a small (theoretical) window for lag right around 3mph where the car still has to disengage, then reengage but at least pack 1 is in the right gear so you save 3rd to 1st switch.
2. Click your Upshifts 3times in a row really fast. Get up to about 4000 rpm in first and just go click click click as fast as you can on the upshift. 1 to 2nd will be super quick, anything after that will feel like an eternity because it does actually take an eternity. See DCTs are only quick when the tranny has already preselected the next gear. So always while acceleration a single upshift will be super quickly, but a double upshift will not be because it takes time to preselect the one gear over.
3. Following the same thinking, you can reproduce the same thing you are seeing when coming to a stop and whacking the gas pedal but completely confusing the tranny about your intentions. While accelerating the DCT preselects the next higher gear
So go in second at really low RPM (where first is still possible), hit the gas while at the sime time you ask the tranny to downshift. It will prb take over 1.5s to do that.
btw of this assumes non-sport mode on. You can see this behavior in sport mode too, but the lag will be smaller because the clutches engage disengage much quicker.
@TX135i - When this happens to you on the freeway its possible that the transition from constant speed to slow acceleration makes the tranny preselect the next higher gear, the deeper pressure on the gas then asks it to downshift and voila you have the lag. This would only make sense if you are not in 7th already.